We are applying conventional ultrastructural (freeze-fracture, thin section, and SEM) techniques to the study of cultured bovine endothelial cells derived from calf aortae and umbilical veins. Variations in gap junctions are being correlated with variations in the capability of the cells to carry out cell-to-cell transfer of small molecules; e.g., ions, tracer dyes, and nucleotides. The in vitro structure and physiology are also being compared with in situ and ex vivo cells to determine the effects, if any, of culturing on the cell junctions. Cocultures of smooth muscle and endothelial cells are also being studied with the structural and physiological techniques.